Prior to the latter half of the 1980's television systems in, e.g., the United States provided only relatively low quality monophonic sound signals. In the latter half of the 1980's BTSC sound signals were incorporated in television signals. The BTSC sound signals include a monophonic signal compatible with earlier standards, and a stereo signal, permitting appropriately equipped receivers to reproduce stereophonic sound. The BTSC signal is an analog FM signal having a carrier frequency at 4.5 MHz (NTSC) above the picture carrier of the television signal.
Advances in digital audio equipment such as compact disc players (CD's) and digital tape recorders (DAT's) have made possible the reproduction of extremely high quality sound reproduction. It is desirable to incorporate such high quality sound reproduction in television receivers. However, due to bandwidth restrictions of existing television broadcast channels it is not possible to transmit sufficient audio information to emulate CD performance. It is possible, however, to incorporate a digital sound signal, albeit of lesser sample rates and resolution than CD signals, in a television signal, which will provide improvements over existing analog sound signals.
P.J.R. Court, in U.S. Pat. No. 3,231,818 describes a television system in which an auxiliary analog sound signal is included in a television signal. The auxiliary signal is an FM signal located in the vestigial sideband of the television signal and is of the same quality as the normal sound signal.
Craig C. Todd in a paper entitled "A Compatible Digital Audio Format for Broadcast and Cable Television", in the IEEE Transactions on Consumer Electronics, Vol. CE-33, No. 3, August 1987, pp. 297-305, describes a method for including a higher quality digital sound signal in a television signal. In this system a digital audio signal, in the form of a quadrature phase shift keying (QPSK) signal, is included in the television signal spectrum above the normal sound carrier. As such the QPSK signal resides at least partially in the vestigial sideband of the next higher adjacent broadcast channel, and tends to violate the bandwidth constraints established for a single broadcast channel.
R.B. Dome, in U.S. Pat. No. 2,635,140 teaches that plural video signals may be encoded in frequency interleaved fashion in the vestigial sideband (and elsewhere) of a video signal and the plural signals may be successfully separated in a receiver. However, the spectrum of audio signals is not conducive to frequency interleaving with video signals.
It is an object of the present invention to provide an auxiliary, high quality digital sound signal, in a broadcast television signal, within the frequency spectrum allotted for a single television signal channel.